The present invention relates to a method and apparatus for shaping glass sheets and particularly those that are shaped by a method involving conveying glass sheets into a shaping station where the glass sheet is delivered at its deformation temperature. The glass sheet is supported between upper and lower shaping molds, the upper one of which being provided with an apertured wall through which suction can be applied to hold the glass sheet in pressurized engagement thereagainst. While the glass sheet is held by suction, a ring-like member is conveyed into the shaping station between the upper and lower molds and the glass sheet is transferred onto the ring-like member that supports the glass sheet for movement through a cooling station where tempering medium, most usually in the form of pressurized blasts of air, is imparted against the opposite surfaces of the glass sheet at a rate sufficient to cool the heated shaped glass sheet and impart at least a partial temper thereto.
In the past, the vacuum molds that supported the glass sheets were either made of a very heavy refractory material or of metal. The lower surface of the vacuum molds, if made of metal, were covered with covers of a flexible refractory material, such as fiber glass cloth and the like, to avoid direct contact between the glass sheet and the bottom surface of the apertured lower wall of the upper mold. Such direct contact would replicate scratches and other defects in the lower mold surface, which would harm the optical properties or deform the glass sheet from the shape desired to be imparted by the upper vacuum mold in the absence of any cover material.
If the upper vacuum mold were made of metal, and the mass production of bent glass sheets was conducted at a rapid rate, the shape of the apertured bottom wall of the upper vacuum mold tended to become distorted. This distorted shape was imparted to the glass sheet. Consequently, bent glass sheets did not conform to the shape desired by the customer. Customer tolerances are quite strict, particularly in the need to apply shaped glass sheets to a frame having a shape desired by automobile stylists so that the shaped glass window forms a continuation of the design of the automobile which includes the curved frame within which the shaped glass window is mounted.
Since the automotive industry has been required in recent years to develop automobiles that reduce their fuel consumption, it has become necessary to bend and temper thinner glass sheets than those found suitable in the past. The present invention relates to the treatment of relatively thin glass sheets, particularly those having a nominal thickness of 1/8 inch (3.2 millimeters) or less. Thinner glass sheets sag more readily than thicker glass sheets at any given elevated temperature above the glass deformation temperature. Hence, it is more difficult to control the shape imparted to thinner glass sheets, and in recent years the shaping of thinner glass sheets has incorporated the use of vacuum molds having lower apertured walls enclosing a chamber through which suction is applied to hold a heat-softened glass sheet by vacuum against the downwardly shaping surface of a shaping mold to control its sag during the shaping operation.
Shaped glass sheets are widely used as side windows or rear windows in vehicles such as automobiles or the like in positions where tempered glass sheets are permitted. To be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side windows and rear windows are installed.
It is also important that the side windows meet stringent optical requirements. In addition, the windows must be free of optical defects that would interfere with the clear viewing therethrough, particularly in their viewing area.
During their fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper as well as to shape the glass sheets. Tempering increases the resistance of the shaped windows to damage resulting from impact. In addition to increasing the resistance of the glass sheet to breakage, tempering also limits any glass sheet fracture to relatively small, relatively smoothly surfaced fragments that are less injurious than the relatively large, jagged fragments that result from the more frequent breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat glass sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel-type furnace where the glass sheet is one of a series of sheets that are heated to the deformation temperature of glass and along an extension of the path into a shaping station where each glass sheet in turn is transferred onto a lifting mold that lifts the glass sheet into adjacency to a vacuum mold. Suction is applied to the vacuum mold to lift and hold the shaped glass sheet in pressurized engagement thereagainst. The lifting mold retracts to below the substantially horizontal path. At about the same time, a ring-like member having an outline shape conforming to that of the glass sheet slightly inboard of its perimeter moves upstream into a position below the upper vacuum mold and above the lower lifting mold. Release of the vacuum deposits the shaped glass sheet onto the ring-like member. The ring-like member conveys the glass sheet into a cooling station for rapid cooling.
In prior art apparatus, materials used for the vacuum molds were either metal or massive refractory materials. When the upper vacuum mold included a downward facing shaping wall of metal that was apertured, intermittent contact with the relatively hot glass sheets caused the downwardly facing wall to distort from the shape desired therefor. The use of flexible fabric materials of a refractory nature such as fiber glass cloth insulated the hot glass sheet from direct contact with the lower metal shaping surface of the upper vacuum mold. However, as mass production rates increased, it was found that the insulation properties of the fiber glass was insufficient to prevent distortion of the upper vacuum mold from its desired shape. As a consequence, the mold developed a distorted shape during the fabrication of approximately 100 shaped glass sheets.
Refractory lower walls were also used in vacuum molds in the prior art. In such constructions, low expansion refractory materials were not so prone to develop distortion in lower apertured walls, but the manner by which the lower refractory walls were supported by the rest of the vacuum mold structure caused structural weakness that led to frequent requirements for vacuum mold replacement.
It would be beneficial for the glass sheet bending art to develop a type of mold that could utilize suction for holding a glass sheet thereagainst in order to impart a more precise shape to the thinner glass sheets presently treated for the production of tempered side windows and rear windows of automobiles and other vehicles than those having metal lower walls that distorted in shape. It would also be beneficial for the glass sheet bending art to develop a vacuum mold of low expansion refractory material that is more durable than prior art vacuum molds of such composition.